\documentclass[12pt]{article}
\usepackage[american]{babel}
\usepackage{siunitx}
\usepackage{booktabs, multirow, longtable, tabu}
\usepackage[onehalfspacing]{setspace}

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\usepackage{showframe}% to show page layout
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\begin{document}

\begin{longtabu} to \textwidth { X[1.5, l] X[1, l]
			*{6}{S[table-format=2.3]}
	}
	\toprule
	 &       & \multicolumn{3}{c}{\textbf{def1}} & \multicolumn{3}{c}{\textbf{def2}} \\
	\cmidrule(lr){3-5}\cmidrule(lr){6-8}
	 &       & {contrast} & {t}  & {p}           & {contrast} & {t}   & {p}          \\
	\midrule
	\endfirsthead
	\toprule
	 &       & \multicolumn{3}{c}{\textbf{def1}} & \multicolumn{3}{c}{\textbf{def2}} \\
	\cmidrule(lr){3-5}\cmidrule(lr){6-8}
	 &       & contrast   & t    & p             & contrast   & t     & p            \\
	\midrule
	\endhead
	\bottomrule
	\multicolumn{8}{r}{continued on the next page}                                   \\
	\endfoot
	\bottomrule
	\endlastfoot
	% end of tabu preamble
	\multicolumn{8}{c}{\textbf{group 1}}                                             \\
	\midrule
	\multirow{6}{*}{\textbf{design1}}
	 & pos1  & 15.05      & 1.06 & 0.145         & -3.41      & -0.28 & 0.610        \\
	 & pos2  & 9.20       & 0.65 & 0.258         & -4.02      & -0.32 & 0.627        \\
	 & pos3  & 32.92      & 2.30 & 0.011         & 4.87       & 0.40  & 0.344        \\
	 & pos4  & 6.46       & 0.69 & 0.247         & 3.94       & 0.45  & 0.327        \\
	 & pos5  & 10.99      & 0.87 & 0.191         & 0.41       & 0.03  & 0.486        \\
	 & pos6  & 36.70      & 2.20 & 0.014         & 30.20      & 1.94  & 0.026        \\
	\midrule
	\multirow{6}{*}{\textbf{design2}}
	 & pos1  & 6.51       & 0.87 & 0.192         & -2.79      & -0.41 & 0.661        \\
	 & pos2  & 1.91       & 0.29 & 0.385         & -3.21      & -0.54 & 0.706        \\
	 & pos3  & 16.49      & 2.52 & 0.006         & 4.21       & 0.73  & 0.233        \\
	 & pos4  & 4.42       & 0.96 & 0.168         & 1.15       & 0.28  & 0.391        \\
	 & pos5  & 0.13       & 0.02 & 0.491         & -0.26      & -0.05 & 0.521        \\
	 & pos6  & 11.73      & 1.63 & 0.052         & 12.18      & 1.88  & 0.031        \\
	\midrule
	\multirow{6}{*}{\textbf{design3}}
	 & pos1  & 17.02      & 1.18 & 0.119         & -5.75      & -0.46 & 0.678        \\
	 & pos2  & 11.08      & 0.77 & 0.220         & -3.85      & -0.30 & 0.618        \\
	 & pos3  & 35.82      & 2.41 & 0.008         & 6.00       & 0.48  & 0.316        \\
	 & pos4  & 9.59       & 0.96 & 0.170         & 1.78       & 0.19  & 0.423        \\
	 & pos5  & 9.25       & 0.70 & 0.243         & 0.86       & 0.07  & 0.472        \\
	 & pos6  & 34.91      & 1.99 & 0.024         & 32.28      & 2.04  & 0.021        \\
	\midrule
	\multicolumn{8}{c}{\textbf{group2}}                                              \\
	\midrule
	\multirow{6}{*}{\textbf{design1}}
	 & pos1  & 16.98      & 1.07 & 0.142         & -10.16     & -0.70 & 0.759        \\
	 & pos2  & 18.47      & 1.10 & 0.136         & 2.77       & 0.18  & 0.429        \\
	 & pos3  & 15.28      & 1.23 & 0.110         & 16.18      & 1.41  & 0.079        \\
	 & pos4  & 21.38      & 1.92 & 0.027         & -0.18      & -0.02 & 0.507        \\
	 & pos5  & 30.56      & 2.54 & 0.006         & 16.49      & 1.50  & 0.067        \\
	 & pos6  & 29.32      & 3.40 & 0.000         & -7.31      & -0.98 & 0.836        \\
	\midrule
	\multirow{6}{*}{\textbf{design2}}
	 & pos1  & 5.45       & 0.71 & 0.239         & -3.97      & -0.56 & 0.712        \\
	 & pos2  & 9.89       & 1.48 & 0.070         & -0.84      & -0.14 & 0.554        \\
	 & pos3  & 7.32       & 1.38 & 0.084         & 4.53       & 0.93  & 0.176        \\
	 & pos4  & 10.15      & 2.04 & 0.021         & -0.86      & -0.19 & 0.575        \\
	 & pos5  & 11.09      & 2.20 & 0.014         & 3.13       & 0.67  & 0.250        \\
	 & pos6  & 9.31       & 1.88 & 0.030         & -3.42      & -0.75 & 0.774        \\
	\midrule
	\multirow{6}{*}{\textbf{design3}}
	 & pos1  & 16.54      & 1.06 & 0.145         & -7.81      & -0.55 & 0.709        \\
	 & pos2  & 11.86      & 0.72 & 0.236         & -6.58      & -0.43 & 0.667        \\
	 & pos3  & 15.75      & 1.29 & 0.099         & 14.89      & 1.32  & 0.093        \\
	 & pos4  & 25.87      & 2.36 & 0.009         & -1.19      & -0.12 & 0.548        \\
	 & pos5  & 30.73      & 2.59 & 0.005         & 14.17      & 1.31  & 0.096        \\
	 & ¸pos6 & 31.61      & 3.66 & 0.000         & -6.28      & -0.85 & 0.802        \\
\end{longtabu}

\end{document}
